Children with cancer are potentially at a high risk of plasma 25-hydroxyvitamin D (25(OH)D) inadequacy, and despite UK vitamin D supplementation guidelines their implementation remains inconsistent. Thus, we aimed to investigate 25(OH)D concentration and factors contributing to 25(OH)D inadequacy in paediatric cancer patients. A prospective cohort study of Scottish children aged 75 nmol/l). In all, eighty-two patients (median age 3·9, interquartile ranges (IQR) 1·9-8·8; 56 % males) and thirty-five controls (median age 6·2, IQR 4·8-9·1; 49 % males) were recruited. 25(OH)D inadequacy was highly prevalent in the controls (63 %; 22/35) and in the patients (64 %; 42/65) at both baseline and during treatment (33-50 %). Non-supplemented children had the highest prevalence of 25(OH)D inadequacy at every stage with 25(OH)D median ranging from 32·0 (IQR 21·0-46·5) to 45·0 (28·0-64·5) nmol/l. Older age at baseline (R -0·46; P<0·001), overnutrition (BMI≥85th centile) at 3 months (P=0·005; relative risk=3·1) and not being supplemented at 6 months (P=0·04; relative risk=4·3) may have contributed to lower plasma 25(OH)D. Paediatric cancer patients are not at a higher risk of 25(OH)D inadequacy than healthy children at diagnosis; however, prevalence of 25(OH)D inadequacy is still high and non-supplemented children have a higher risk. Appropriate monitoring and therapeutic supplementation should be implemented.